The invention relates to a method for switching off a switched-mode power supply in the event of a short-circuit, and to a switched-mode power supply. In the switched-mode power supply, an AC voltage is applied to AC voltage inputs of a bridge rectifier to whose DC outputs a parallel circuit containing an electrolytic capacitor and a series circuit formed of a primary winding of a transformer and of a controllable switch is connected. A control input of the controllable switch is connected to the output of a control unit, and in which a series circuit containing a first diode and a first capacitance is connected in parallel with the secondary winding of the transformer.
Switched-mode power supplies that produce a regulated output voltage are equipped with a transformer to whose primary winding the DC voltage outputs of a rectifier, for example a bridge rectifier, can be connected by a controllable switch which is switched by a control unit or by a regulator. A series circuit containing a diode and a capacitance across which the regulated output voltage can be tapped off is connected in parallel with the secondary winding of the transformer. The controllable switch is opened and closed by switching pulses produced by the control unit or by the regulator, depending on the load on the output of the switched-mode power supply.
By virtue of their construction, switched-mode power supplies are resistant to short-circuits on their output. It is known for the primary current through the controllable switch and through the primary winding to be measured by a shunt resistor in order to ensure short-circuit protection on the primary side, as well.
A disadvantage of such short-circuit protection is that losses occur in the shunt resistor.
It is accordingly an object of the invention to provide a method for switching off a switched-mode power supply in the event of a short-circuit, and a switched-mode power supply which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, in which only a negligibly small power loss occurs.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for switching off a power supplying device in an event of a short-circuit. The method contains the steps of providing a switched-mode power supply including:
a bridge rectifier having AC voltage inputs receiving an AC voltage and DC outputs;
an electrolytic capacitor;
a transformer having a primary winding and a secondary winding;
a first series circuit containing the primary winding of the transformer and a controllable switch having a control input, the first series circuit is connected in parallel with the electrolytic capacitor and together define a parallel circuit connected to the DC outputs of the bridge rectifier;
a control unit having a control input and an output connected to the control input of the controllable switch; and
a second series circuit containing a diode and a capacitance and connected in parallel with the secondary winding of the transformer; and
opening the controllable switch if a voltage drop across the electrolytic capacitor falls below a predetermined threshold value.
With regard to the method, the controllable switch is opened when the voltage drop across the electrolytic capacitor falls below a threshold value that can be predetermined.
With the foregoing and other objects in view there is further provided, in accordance with the invention, a switched-mode power supply. The switched-mode power supply contains a bridge rectifier having AC voltage inputs for receiving an AC voltage and has DC outputs. An electrolytic capacitor and a transformer having a primary winding and a secondary winding are provided. A first series circuit containing the primary winding of the transformer and a controllable switch having a control input is provided. The first series circuit is connected in parallel with the electrolytic capacitor and together define a parallel circuit connected to the DC outputs of the bridge rectifier. A control unit having a control input and an output is connected to the control input of the controllable switch. A second series circuit containing a diode and a first capacitance and is connected in parallel with the secondary winding of the transformer. A capacitive voltage divider formed of a second capacitance, a center tap, and a third capacitance, and the capacitive voltage divider is connected in parallel with the electrolytic capacitor. A comparator having a first input is connected to the center tap of the capacitive voltage divider, a second input is to be connected to a reference voltage, and an output is connected the control input of the control unit.
With regard to the circuit, the object is achieved in that a capacitive voltage divider containing the second and the third capacitance is connected in parallel with the electrolytic capacitor, and that the center tap of the voltage divider is connected to the first input of the comparator. The second input of the comparator receives a reference voltage and the output of the comparator is connected to the control input of the control unit.
The invention is based on the knowledge that an electrolytic capacitor represents a series circuit containing an equivalent series resistance, an inductance and the actual capacitance. Since the primary current rises in the event of a short-circuit, the voltage across the electrolytic capacitor falls owing to the voltage drop across its equivalent series resistance. The voltage across the electrolytic capacitor is compared in a comparator with a reference voltage, which is used as a threshold value. If the voltage across the electrolytic capacitor falls as a result of a short-circuit, irrespective of the point at which the short-circuit is located, to the threshold value, then the controllable switch is opened, and the switched-mode power supply is thus switched off.
The invention therefore represents perfect short-circuit protection for a switched-mode power supply since it switches the switched-mode power supply off when a short-circuit occurs at any point. A major advantage of the invention is that there is no longer any need for a shunt resistor, which produces losses. The short-circuit protection according to the invention operates with virtually no losses.
In accordance with an added feature of the invention, a resistor is connected between the second capacitance and the third capacitance of the capacitive voltage divider.
In accordance with an additional feature of the invention, a further resistor having a first terminal is connected to the first input of the comparator and a second terminal is to be connected to a voltage for applying the voltage to the first input of the comparator. A further diode is connected in parallel with the second capacitance of the capacitive voltage divider that is also connected to the first input of the comparator.
In accordance with another feature of the invention, a fourth capacitance is connected in parallel with the controllable switch.
In accordance with a further feature of the invention, a bistable multivibrator is connected between the output of the comparator and the control input of the control unit.
In accordance with another added feature of the invention, the bistable multivibrator is a flipflop.
In accordance with a further additional feature of the invention, the comparator, the bistable multivibrator and the control unit form an integrated circuit.
In accordance with a concomitant feature of the invention, the controllable switch is a field-effect transistor.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for switching off a switched-mode power supply in the event of a short-circuit, and a switched-mode power supply, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.